Helicobacter pylori is a bacterial pathogen that infects the gastric mucosa of 50% of humans worldwide and elicits gastritis that can progress to peptic ulcers or gastric cancer, which accounts for more than 650,000 deaths each year. No vaccine is available to prevent or treat H. pylori infection, and antibiotic resistance is an ever-increasing problem that undermines treatment efficacy. A distinguishing feature of H. pylori infection is the chronic, polymorphonuclear leukocyte (PMN, neutrophil)-dominant inflammatory response. Patient biopsy samples demonstrate that PMNs reach the mucus layer over the gastric epithelium and engulf large numbers of bacteria in this locale. However, H. pylori is not killed and NADPH oxidase-derived reactive oxygen species (ROS) released into the extracellular milieu damage host tissue. Despite the central role of neutrophils in H. pylori pathogenesis, our understanding of bacteria-PMN interactions is rudimentary. Thus, we undertook this study to address critical knowledge gaps regarding the phenotype and fate of infected PMNs and their bacterial cargo. To this end, we created a collection of isogenic bacterial mutants that lack major virulence factors alone or in combination, and also exploited recent discoveries that have revolutionized our understanding of the role of neutrophils in the immune response, as indicated by their immunomodulatory capacity and ability to undergo subtype differentiation in vivo. Our central hypothesis is that H. pylori exploits PMN phenotypic plasticity as part of its virulence strategy. Consistent with this, we present extensive, convincing preliminary data to suggest that H. pylori-neutrophil interactions are significantly more complex than previously appreciated. Thus, we propose that at early stages of infection H. pylori evades killing by manipulation of phagosome maturation and granule targeting, and at the same time induces PMN subtype differentiation. Constitutive PMN apoptosis appears profoundly impaired, and ~70% of neutrophils remain viable at 72 hours. Ultimately, infected cells appear to die not by delayed apoptosis, but rather by an alternative mechanism, and this is accompanied by the onset of robust extracellular H. pylori growth. To test this infection model we will in three specific aims analyze bacterial trafficking and degranulation, define changes in PMN phenotype and functional capacity, elucidate the effects of H. pylori on PMN lifespan and mechanism of cell death, and begin to define the roles of major bacterial virulence factors in these aspects of virulence. Methods utilized will include but are not limited to super-resolution confocal microscopy, electron microscopy, RNA-Seq, and high-throughput analysis of cytokine production using Fluidigm microfluidic chips. Completion of this study will be an essential first step toward the development of new therapeutics that modulate PMN function as a means to control infection.